As a switching power supply circuit for generating a predetermined direct current output voltage from a direct current input voltage, a DC/DC converter using a synchronous rectification system is known. This type of DC/DC converter compares a feedback voltage, which is based on a direct current output voltage supplied as feedback from the output side, with a predetermined reference voltage and, based on the results of the comparison, alternately turns on/off a main switching element (switching element on the power supply side) and a subordinate switching element (switching element on the ground side) to convert a direct current input voltage into the predetermined direct current output voltage.
With the DC/DC converter of the above-mentioned type, there may arise a so-called backflow phenomenon, a phenomenon in which an output current to be supplied normally to a load flows into the subordinate switching element side via a coil. Such a backflow phenomenon is apt to occur particularly under a light load, and has to be prevented from happening, because it presents a cause of a reduction in the efficiency of the DC/DC converter. In a DC/DC converter according to a conventional technology, therefore, a circuit for preventing coil current reversal during synchronous rectification is provided, see Japanese Patent Document JP-A-2004-328975.
The circuit for prevention of coil current reversal during synchronous rectification according to the conventional technology compares the current flowing in the coil with a finite current value or 0 mA for preventing reversal to turn off the subordinate switching element for synchronous rectification, thereby preventing the polarity of the coil current from inverting. The circuit also adopts a so-called zero comparator system which monitors the change of a terminal voltage VLX on the subordinate switching element side of the coil from a negative voltage to a positive voltage and, immediately after reversal of the coil current, turns off the subordinate switching element for synchronous rectification.
Preventing the reversal of the coil current under any conditions, however, has required an extensive circuit scale and, at the same time, has driven a high speed comparator, thus increasing current consumption. These facts have constituted factors impeding the speed increase of the switching frequency in recent years.
The present invention has been accomplished in the light of the above-mentioned earlier technologies. It is an object of the invention to provide a switching power supply circuit which can achieve the prevention of coil current reversal, while minimizing current consumption and speeding up the switching frequency, without requiring a quick response.